1. Field of the Invention
The invention relates to the design of a tapered electrochemical cell having an electrode formed of a bed of electrochemically active particles through which an electrolyte circulates. In particular, the dimensions of the cell cavity promote bridging of particles across the cavity and formation of voids to maintain a highly permeable bed.
2. Description of Related Art
Metal/air batteries, such as zinc/air batteries, have long held promise for application in stationary and mobile power sources because of their high energy density, low cost, and use of benign materials. Zinc/air batteries, for example, typically are made of zinc: (as an anode material), an electrolyte solution (usually alkaline NaOH or KOH), and a gas-diffusion electrode. The gas diffusion electrode is typically a porous structure through which atmospheric oxygen diffuses to an electrolyte-wetted and catalyzed surface, where oxygen is reduced ultimately to hydroxide. Such gas diffusion electrodes can be made of porous carbon particles bonded to a supporting and conducting positive screen, catalyzed with noble metals or organo-metallic compounds, and wet-proofed on the air side with fluorocarbon materials.
Secondary batteries, as described in U.S. Pat. No. 4,842,963 by Ross, may use a zinc electrode supported on a porous carbon substrate, which is alternately charged and discharged by electrochemical means. Primary batteries use a consumable solid or particulate zinc anode and are not generally recycled. LUZ, Inc., has constructed a zinc/air fuel battery wherein cassettes containing zinc particles are discharged in a vehicle, then removed and recycled at a fixed industrial site. (See J. R. Goldstein and B. Koretz, Tests of a Full-Sized Mechanically Rechargeable Zinc-Air Battery in an Electric Vehicle", Paper No. 93410, Proceedings of the 28th Intersociety Energy Conversion Engineering Conference, American Chemical Society, Atlanta, Ga., Aug. 8-13, 1993, p. 2.279)
A zinc/air fuel battery using particles of zinc added from outside may provide propulsion energy for an electric vehicle. Zinc particles and alkaline solution could be added periodically, in a manner analogous to refueling an automobile. At the same time, the spent electrolyte (containing zincate and zinc oxide) produced during previous discharge would be removed from the vehicle. The zinc metal and fresh electrolyte would be regenerated from this spent electrolyte by electrolysis and/or various chemical or thermal reduction techniques at a fixed industrial or service site. Thus, a fixed quantity of zinc would be recycled indefinitely between recovery equipment and the vehicle, and the vehicle could be refueled rapidly at a service station for indefinite range extension. Such an application is particularly attractive in fixed route vehicles, such as vans and buses, which may be refueled periodically at a single site on the route.
Conventional zinc/air fuel batteries developed for electric vehicles are broadly classified as either "fluidized bed" or "static bed" systems. Zinc particle/air batteries developed for vehicles often make use of a fluidized or slurried suspension of zinc particles in an electrolyte. According to known solid flow behavior, random close packing of solid spheres into a fluid-filled cavity results in a bed of 63% solid fraction. Frictional spheres will reduce this solid fraction to about 58-63%. If parallel walls of the cavity are separated by 2.53 diameters of the sphere, then the packing density falls to about 49 or 53%, respectively. This substantial reduction in solid fraction (by 20% of close packing figure) greatly reduces hydraulic resistance of the bed.
Backhurst et al. (U.S. Pat. No. 3,879,225) teaches an electrode arrangement for an electrochemical cell which uses an upwards flowing electrolyte to expand and to fluidize a zinc particle bed; the particles make momentary contact with a negative current collector during discharge. Backhurst shows that a tapered cell bottom provides a point of electrolyte entry giving the electrolyte sufficient velocity to fluidize the bed, while permeable membrane walls and lower velocities in the bulk of the cell prevent loss of particles from the cell.
Doniat et al. (U.S. Pat. No. 3,887,400) provides a method for the use of a fluidized suspension of zinc particles that make momentary and intermittent contact with a negative (anode) current collector, then reside in the bulk of the slurry out of contact with the negative while reaction products diffuse away from the zinc particle surface. Doniat et al. (U.S. Pat. No. 3,981,747) further provides a means for continuous recharge of a slurried suspension of zinc particles by periodic contact with a more electronegative metal (e.g., aluminum), followed by subsequent discharge of the zinc by momentary contact with an anodic current collector.
Solomon et al. (U.S. Pat. No. 4,147,839) teaches a means for fluidizing a bed of zinc powder by use of impellers located within each cell and various methods for transfer of such slurried zinc particles to and from the cell under influence of a pressure difference ("hydraulic transfer"). In addition to dense zinc particles, the zinc may reside as a coating on inert particle cores (Doniat, U.S. Pat. No. 4,126,733); fluidized bed discharge is similar to that for dense particles as long as the cores remain coated.
Fluidized bed systems have several disadvantages. The continuous fluidization to overcome particle settling consumes energy, up to 5% of the gross output of the cell. Also, very small particles are required to provide high power densities (W/cm.sup.2 of cell cross-sectional area) due to the short contact time between the particles and the collector and the necessity to maximize the area/volume ratio of the particles. The pumping of the fluidized bed between series-connected cells requires a substantial pressure drop and allows a shunt current to flow through the suspension; Jacquelin, however, teaches a method of ameliorating this current (U.S. Pat. No. 4,038,458). Finally, the fluidized zinc particles tend to abrade the fragile interelectrode separator, the current collectors, and other cell components, requiring more robust structures with high electrical resistance or increased weight.
To avoid these problems, Evans et al. (U.S. Pat. No. 5,006,424) teaches the use of a "static" bed anode consisting of zinc particles, through which electrolyte is allowed to flow by natural convection, driven by the density difference between the electrolyte in the pores of the bed and the electrolyte in a circulation duct external to the bed. A concentration difference develops during discharge, which results in the build-up of dissolved zincate and suspended solid reaction products in the interstices between the zinc particles. Because the bed is not fluidized, mass transport requirements of the cell can be met without the use of mechanical pumps to levitate the bed. The energy loss for transport is very small as long as the bed maintains a low hydraulic resistance. Either solid particles or metal-coated inert cores can be used.
Developers at LUZ, Inc. have disclosed several versions of zinc particle anodes configured as static beds. Goldstein et al. (U.S. Pat. No. 5,145,752) discloses a means for using very fine zinc particles (5-500 .mu.m) in an alkaline electrolyte with a gas diffusion electrode. Brokman et al. (U.S. Pat. No. 5,185,218) further shows the use of a particulate zinc anode with a cathode depolarized by an oxygen-bearing liquid such as a fluorocarbon oil. Goldman et al. (U.S. Pat. No. 5,208,526) teaches the use of a static bed anode which surrounds an air cathode unit. In the practical vehicle battery (Goldstein and Koretz, ibid.), only 80% of the zinc in the cell is available for use, and the cassette containing the zinc must be refurbished at an industrial site in a process which recovers unconsumed zinc, zinc oxide, zincate and electrolyte. If the bed clogs with a paste of unreacted zinc, zinc oxide, zincate, and electrolyte, then high velocity electrolyte jets can be used to dislodge and remove caked material from the cell in order to recharge the cell mechanically.
In general, packed or static bed anodes have fundamental disadvantages. As the bed is consumed by anodic dissolution, the particles and interstices between particles become smaller, and hydraulic resistance increases inversely with a power function of particle size. At a fixed discharge rate, the bed eventually clogs, forming a thick paste of unconsumed zinc, electrolyte, and solid reaction products. In practice, often only about 50-80% of the zinc can be used before the entire bed and reaction products must be removed to prevent caking. If zinc coated cores are used in a static cell, the bed does not necessarily cake, but inert cores must be removed prior to mechanical recharge. The current distribution is highly non-uniform, causing some particles to be depleted of zinc before 100% consumption of the zinc in the bed.
Finally, for both dense particles and coated cores, the static bed cannot be replenished continuously under load from without (to match the rate of discharge), because of changing bed porosity and accumulation of inert cores, respectively. This requires that the total mass of zinc be located within the cell. For large vehicle applications, the zinc loading (&gt;12 kg Zn/m.sup.2) requires that the cell be excessively thick and heavy, and thereby possibly damaged by road shocks.
It would be desirable to provide a battery that has full (100%) consumption of the added particles and which maintains low resistance and does not clog or cake. Further, it would be advantageous to add particles to the bed by gravity feed from a hopper or by pumped transport as a slurry. Finally, the cell should allow electrolyte circulation through the bed for heat and mass transport with very low hydraulic power requirements.